This invention relates to a web-fed printing press having at least two printing units in a row for printing an image whose dimension in the longitudinal direction of the web is longer than the circumference of the plate cylinder of each unit. The invention also deals with a method of printing such images on the web without intervening blanks between the printed images.
Printing firms are sometimes requested by clients to produce printings as large as, say, from 70 inches (1778 millimeters) to 90 inches (2286 millimeters) in top-to-bottom dimension (i.e., dimension determined by the circumference of the printing cylinder, or dimension in the longitudinal direction of the web on which the printings are made). For offset printing of such large images, the most widespread conventional practice has been to prepare and keep in stock an assortment of outsize plate cylinders, as well as blanket cylinders to match, with a diameter of 23.6 inches (600 millimeters) and thereabouts, for various sizes of printings to be made. Such outsize cylinders have been interchangeably mounted to printing presses as the need arises.
This conventional practice is objectionable by reason of the outsize plate cylinders and blanket cylinders themselves and, of course, of the presses of matching size required by such cylinders. All such equipment demand inordinately high costs for manufacture, installation, operation, and storage.
Another known method involves the use of a relief or letterpress printing plate in the form of an endless belt running over a plate cylinder and a guide roller or rollers. The plate cylinder is, in fact, a sprocket having teeth for positive engagement in series of perforations formed in the side margins of the endless belt. The elongate image is imprinted from the belt on to the web running against an impression cylinder.
The relief printing belt does, however, possess the weakness of being not so satisfactory in the quality of printing as that by offset printing. The printings made by this prior art method is also unsatisfactory in positional accuracy as the plate takes the form of an endless belt and is driven by the sprocket. The service life of the printing belt is questionable, too, by reason of the presence of the perforations in its side margins.
The listed drawbacks of the two foregoing devices are altogether absent from a more advanced printing system according to Japanese Unexamined Patent Publication No. 63-189633. This patent application suggests use of two or more printing units in serial arrangement for printing on a web running successively therethrough. As the printing units prints different sections of an image at prescribed spacings on the web, the image sections provide in combination the image of any desired top-to-bottom dimension up to the sum of the circumferences of all the plate cylinders in use.
This third prior art approach, though definitely more favorable than the first two, has its own shortcomings. It had, first of all, limitations in the top-to-bottom dimension of the printings to be made. Since the plate cylinders of all the printings units were in constant rotation at the same peripheral speed as the running speed of the web, the spacings left unprinted by each printing unit were each equal to the circumference of each plate cylinder, or to an integral multiple thereof. The top-to-bottom dimension of each image thus printed, constituted of the sections printed by the respective units, was therefore limited to the total of the circumferences of all the plate cylinders if no blanks were to be left on the web.
If, on the other hand, images were to be printed whose top-to-bottom dimensions were less than the total of the circumferences of the plate cylinders, blanks were unavoidably created between the printed images on the web, each blank being equal to the difference between the total circumferential dimension of the plate cylinders and the image dimension in the longitudinal direction of the web. The blanks are nothing less than a waste of paper. Moreover, they necessitated the additional post-printing operations of cutting off the blanks and disposing of the cuttings.